Vehicular truck tires often have rubber treads of a rubber composition comprised of a combination of natural cis 1,4-polyisoprene rubber and polybutadiene rubber with carbon black reinforcing filler which may also contain silica as an additional reinforcing filler.
Historically, it has heretofore been proposed to replace the natural rubber of the tread rubber composition with a specialized and/or designated polybutadiene rubber. For example, see U.S. Pat. No. 7,956,146.
While such natural rubber-rich rubber treads often contain a small amount of synthetic cis 1,4-polybutadiene rubber to promote resistance to tread wear during use of the tire in heavy duty service, total replacement of the natural rubber with synthetic cis 1,4-polybutadiene rubber was considered to be a significant departure from past practice at that time.
A synthetic cis 1,4-polybutadiene rubber having a broad heterogeneity index was proposed for such purpose.
The cis 1,4-polybutadiene rubber was considered to be specialized in a sense of differing from a more conventional cis 1,4-polybutadiene elastomers by having a broad heterogeneity index, namely a significant difference between its number average molecular weight (Mn) and weight average molecular weight (Mw) in which the number average molecular weight (Mn) is relatively low for a cis 1,4-polybutadiene rubber. Such combination of wide heterogeneity index and relatively low Mn for the specialized cis 1,4-polybutadiene elastomer is believed to be indicative of a branched configuration for the elastomer, namely that the polymer structure has a degree of branching which, in turn, is believed to be responsible for better processability of the specialized cis 1,4-polybutadiene to thereby also enable the total replacement of the natural rubber for the tire tread on an uncured rubber processability basis.
For each of the outer tread cap and underlying sub tread cap of the stratified tread, it is proposed to provide rubber compositions comprised of a combination of natural cis 1,4-polyisoprene and dual cis 1,4-polybutadiene rubbers together with reinforcing fillers comprised of a combination of rubber reinforcing carbon black and precipitated silica. For the polybutadiene rubber, it is proposed to use a high performance polybutadiene rubber, referred to herein as a designated cis 1,4-polybutadiene rubber, to promote stiffness and abrasion resistance for the tread rubber composition. However, it is seen that such rubber is difficult to process because of its high uncured viscosity (Mooney viscosity) without significant addition of rubber processing aids which is further seen to unbalance one or more physical properties of the rubber composition. Therefore, it is desired to evaluate providing an additional more processable cis 1,4-polybutadiene rubber for the outer tread cap and sub cap components of the stratified tread (referred to herein as a specialized cis 1,4-polybutadiene rubber) together with natural cis 1,4-polyisoprene rubber to substantially maintain various cured physical properties of the rubber composition containing a combination of precipitated silica and carbon black while promoting better processability of the uncured rubber composition.
In the description of this invention, terms such as “compounded rubber”, “rubber compound” and “compound”, if used herein, refer to rubber compositions containing of at least one elastomer blended with various ingredients, including curatives such as sulfur and cure accelerators. The terms “elastomer” and “rubber” may be used herein interchangeably unless otherwise indicated. It is believed that such terms are well known to those having skill in such art.
The glass transition temperature (Tg) of the solid elastomers and liquid polymer may be determined by DSC (differential scanning calorimetry) measurements, as would be understood and well known by one having skill in such art. The number average molecular weight (Mn) of the solid elastomers and liquid polymer may be determined by GPC (gel permeation chromatography) measurements as would be understood and well known by one having skill in such art.